1. Technical Field
The present invention relates generally to a steering system for a four wheel steering vehicle. More particularly, the invention relates to a system for controlling the steering angles of front and rear wheels according to a steered angle of a steering wheel to enhance traveling stability when turning.
2. Background Art
A steering control system is well known in the art which includes a front power steering unit having a control valve for switching a direction of working fluid supplied to one side or another of a piston in a power cylinder for turning front wheels synchronously with the rotation of a steering shaft to provide assistance in steering the front wheels to the right or the left.
Recently, a steering rear wheels has been developed. This system further includes a rear power steering unit, as well as a front power steering unit, each having a control valve for controlling pressure of working fluid directed to a power cylinder dependent upon the lateral acceleration of a vehicle when turning, switching a direction of the working fluid applied to one or the other sides of a piston in the power cylinder according to vehicle speed. This control valve is operable to control the power cylinder for actively steering the rear wheels. The system requires separate pumps for building up working fluid pressure for directing working fluid to the front and rear power steering units.
However, the above prior art system requires separate hydraulic control circuits with independent pumps therefor for steering the front and rear wheels respectively. This results in a bulky system. In a case where such pumps are driven by engine power, it imposes an added load on the engine causing it to be slowed down. Further, space for accommodating the system is limited and thus installation thereof is difficult.